Project Summary/Abstract. The public health burden due to Lyme disease continues to expand in the eastern United States. New interventions to reduce risk are needed to complement well-known methods such as acaricidal applications or personal protection. Reservoir targeted approaches such as oral vaccination, feed through acaricides or genetic modifications are currently under development or are being field tested. These newer strategies focus on the white footed mouse, Peromyscus leucopus, which is widely regarded as the main host for subadult deer ticks (Ixodes dammini or I. scapularis), the vector for Lyme disease and diverse other zoonotic infections in the northeastern U.S. Indeed, this mouse has been incriminated as the main northeastern U.S. reservoir host for the agent of Lyme disease (Borrelia burgdorferi) as well as for that of human babesiosis (Babesia microti) and deer tick virus encephalitis. However, the evidence that P. leucopus is the main host for subadult deer ticks is largely descriptive using indirect extrapolations and comprise limited observations from a small number of sites. The ideal means of incriminating a reservoir host is to determine the source of the bloodmeal that served to infect the vector but assays that have been described to date for ticks have been poorly reproducible or insensitive. Our overarching goal for this proposed project is to provide a means to better understand natural variability of the relative contribution of other host species on the transmission of the agent of Lyme disease. This is required to evaluate the costs and benefits of mouse targeted interventions. We have determined that host retrotransposons may be targeted by PCR assays and provide specific signatures of the bloodmeal taken by a tick in its previous stage, and now seek to refine and validate this mode of analysis. Accordingly, we seek to (1) develop and validate a sensitive, specific, simple and reproducible retrotransposon assay for identifying the remnants of host bloodmeals in host-seeking nymphal deer ticks; (2) use this assay to determine whether the Peromyscus blood index (PBI) may vary across sites and within sites over time; and (3) experimentally confirm the PBI by undertaking interventions targeted to mice and assaying host seeking nymphal deer ticks thereafter to determine whether there has been a shift in hosts. Although our immediate goal is to provide a solid rationale for mouse targeted interventions against Lyme disease, the assay itself could be adapted to better understand the mode of perpetuation of diverse other tick-borne zoonoses.